Email filtering firm MessageLabs blocked more than 54,000 copies today. Natasha Staley, a consultant with the company, said: "It's out there and spreading but it's nowhere on the big as MyDoom."

Tomorrow morning (March 2) between 06:0am and 8:59am the worm will try to activate PC speakers into making a constant beeping noise. It's unclear if this is simply more mischief or an attempt to push infected users into cleaning up their PCs.

Like its siblings, Netsky-D tries to make registry-key changes, including some that are de-activate any lurking copies of MyDoom-A and MyDoom-B viruses.